1. Field of the Invention
This invention relates to digital subscriber line transceivers for communicating over telephone subscriber loops, and more particularly, to performing equalizer training for asymmetric digital subscriber line transceivers under TCM-ISDN crosstalk environment.
2. Description of the Related Art
The Telecommunications Standards Section of the International Telecommunication Union (sometimes designated as ITU-T) provides recommendations to facilitate the standardization of the telecommunications industry. Two of these recommendations are referred to as G.992.1 and G.992.2. Recommendation G.992.1 refers to an asymmetric digital subscriber line (ADSL) transceiver that is an ADSL industry standard for network access at rates up to 8.192 mbit/s downstream (toward subscriber) and 640 kbit/s upstream (toward central office or network administrator). Recommendation G.992.2, on the other hand, refers to an ADSL transceiver that is a lower data rate version of a G.992.1 ADSL transceiver. Bit rates up to 1.5 mbit/s in the downstream direction and 512 kbit/s upstream are possible with this standard. Factors such as the electrical characteristics of the customer""s equipment, the distance between the subscriber and central office, and the error rate allowed all contribute to the bit rate of the G992.1 and G992.2 transceivers.
The G.992.1 and G.992.2 standards are similar to one another in that they both use the discrete multi-tone (DMT) modulation technology. DMT modulation employs multiple carriers for both upstream and downstream communication. Frequency-division-multiplexing is often used, where upstream and downstream communication use carriers in different frequency bands, while spectrum overlapping is also possible by using echo cancellers. By working with a large number of carriers rather than a single carrier, the available channel capacity is maximized thereby optimizing performance of the transmission. Once a connection between the central office side and the customer side has been established, the transceivers continue to monitor the changing signal-to-noise ratios on the line and swaps bits from one carrier to another to maintain system performance. One of the major differences between the standards is that G.992.1 downstream communication uses carriers up to index 255, while G.992.2 downstream communication uses carriers only up to index 127. Thus, standard G.992.2 has a smaller available downstream bandwidth than standard G.922.1.
Both G.992.1 and G.992.2 have an Annex C that defines a special recommendation for ADSL transceivers under the time compression multiplexingxe2x80x94integrated services digital network (TCM-ISDN) noise environment. Each of these respective Annexes is herein incorporated by reference in its entirety. Reference to xe2x80x9cAnnex Cxe2x80x9d may be to either or both of G.992.1 and G.992.2. TCM-ISDN is defined in ITU-T recommendation G.961, Appendix III. This recommendation is also herein incorporated by reference in its entirety. Recommendation G.961 Appendix III uses a ping-pong scheme to alternately transmit data streams in two directions under the control of a 400 Hz clock referred to as the TCM-ISDN timing reference (TTR). G.992.1 and G.992.2 Annex C transceivers also transmit signals based on this TTR signal. For example, the ATU-C (central office transceiver) transmits the data stream in the first half of the TTR period, and the ATU-R (customer transceiver) transmits basically in the second half of the TTR period.
Recommendations G.992.1 and G.992.2 define the ADSL transceiver unit on the central office side (referred to as xe2x80x9cATU-Cxe2x80x9d), and the ADSL transceiver unit on the remote terminal or customer premises side (referred to as xe2x80x9cATU-Rxe2x80x9d). Although each recommendation specifies the transmitter portion of the transceiver, the receiver portion of the transceiver is left undefined. Thus, transceiver designers have a significant amount of discretion in designing this receiver portion.
In designing the receiver portion of a transceiver, one must consider how to correct for channel distortion and timing errors. Crosstalk interference between the various wire-pairs bundled together in a conventional copper transmission line is a major contributor to channel impairment. Generally, crosstalk noise belongs to one of two groups: near end crosstalk (NEXT) and far end crosstalk (FEXT). NEXT is the crosstalk caused by signals in adjacent lines transmitted from the same end of the cable, while FEXT is the crosstalk caused by signals in adjacent lines transmitted from the remote end of the cable. NEXT is typically much stronger than FEXT. Annex C transceivers synchronize the transmission and reception to the TCM-ISDN timing reference so that the receiver receives data signal in FEXT time where the signal-to-noise ratio is higher, while it receives no data signal or data signal at a lower data rate in NEXT time where the signal-to-noise ratio is low.
The ADSL symbols under FEXT noise are called FEXT symbols, while all the other symbols are called NEXT symbols. As such, the ADSL transceivers function under a TCM-ISDN crosstalk noise environment having a level that changes periodically. The crosstalk noise level during the FEXT symbols is quite low, so the data can be transferred at a higher rate. The crosstalk noise level during the NEXT symbols, however, is high. Thus, while functioning under the NEXT symbols, the data transmission may be at lower rate (e.g., in a dual bitmap mode), or paused (e.g., in a FEXT bitmap mode, also referred to as a single bitmap mode).
An equalizer can be used to correct for channel distortion. Typically, the receiver portion of a transceiver is comprised of, among other components, an equalizer. Typically, such an equalizer is comprised of: (1) a time-domain equalizer (TEQ), and (2) a number of frequency-domain equalizers (FEQ). The TEQ compensates the channel distortion in the time domain and eliminates inter-symbol interference before the signal is converted into the frequency domain through a Fast Fourier Transform (FFT). After FFT, there is one FEQ for each carrier (sub-channel) employed in the DMT modulation scheme. Because the recommendations are silent as to the receiver portion of the transceiver, they are also silent to equalizer implementation and training. As such, there are no known techniques for performing equalizer training in ADSL Annex C transceivers.
What is needed, therefore, is a technique for training an equalizer of an ADSL Annex C transceiver. The equalizer must be trained so as to achieve optimal communication channel performance. The technique must define what periods during the transmission sequence the equalizer should be trained. Additionally, the technique must define what symbols should be used for training. The technique must substantially eliminate inter-symbol interference, and neutralize the effect of strong cross-talk noise (e.g., NEXT).
The present invention provides a technique for training an equalizer of an ADSL Annex C transceiver. The equalizer is trained so as to achieve optimal communication channel performance. The technique defines the periods in the transmission sequence at which equalizer training takes place. Additionally, the technique defines what symbols comprising the transmission are used for training. The technique neutralizes the effect of strong cross-talk noise and substantially eliminates inter-symbol interference.
Once a communication link is established between two ADSL Annex C transceivers, the channel distortion associated with that link is compensated for by performing equalizer training in the receiver sections of both transceivers. This equalizer training is accomplished, in part, by sending REVERB signals from the transmitter of one transceiver to the receiver of the remote transceiver. During a TEQ training phase, the cyclic prefix is not introduced. In accordance with the present invention, the TEQ is trained only on FEXT symbols except the FEXT symbols adjacent to NEXT symbols. Thus, the signals received during NEXT symbols and the FEXT symbols adjacent to NEXT symbols are not used for TEQ training. In an early phase of FEQ training, the cyclic prefix is not introduced. The FEQ is also trained on FEXT symbols only except the FEXT symbols adjacent to NEXT symbols.
During a late training phase after the cyclic prefix is introduced, the training of the FEQ is continued by using signals received on all FEXT symbols only. Due to the introduction of the cyclic prefix and the time-domain equalizer, the inter-symbol interference is basically eliminated. Therefore, the FEXT symbols adjacent to NEXT symbols are not affected by the TCM-ISDN noise that exists in NEXT symbols only. As a result, the received signal on those symbols can also be used to train the FEQ. p In the SHOWTIME phase, the FEQ can be updated on FEXT and NEXT symbols for channels where the number of bits per symbol is non-zero. Since those channels are used for data transmission, their respective FEQ""s need to be updated so that those FEQ""s can keep tracking the channel changes. Also in the SHOWTIME phase, the adaptation of the FEQ is performed because the signal to noise ratio at those channels is sufficient. Otherwise, those channels would not have been selected for data transmission. Only one FEQ per communication sub-channel is used when operating in a FEXT bitmap (FBM) mode or in a dual bitmap (DBM) mode.
By employing the present invention, the performance of the TEQ as well as the training and the later adaptation of the FEQ, are not affected by the strong NEXT interferences existing in the TCM-ISDN. The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter.